1. Field of the Invention
The present invention relates to an optical encoder for detecting the revolution velocity of an electric motor, and the velocity, the position, the direction of movement, etc., of a rotating portion or a linearly moving portion of various kind of devices.
2. Description of Prior Art
Conventionally, a rotary encoder has been used as means for detecting the revolution velocity, etc., of a rotary mechanism portion of a motor or other various kinds of devices by using a pulse signal, and a linear encoder has been used to detect the velocity, the position, or the like, of a linearly moving member also by using a pulse signal.
A conventional optical rotary encoder is shown in FIG. 1 (front view) and FIG. 2 (right side view). In this rotary encoder, a code disk 2 is fixedly attached on a rotary shaft 1. This rotary shaft 1 is coupled to an object to be measured so that the code disk 2 is rotated together with the object to be measured. As seen in FIG. 1, a plurality of detection holes 2a are bored in the code disk 2. These detection holes 2a are formed through the code disk 2 and circumferentially continuously arranged at a predetermined circumferential distance. The detection holes 2a are actually continuously formed along the entire circumferential area in the code disk 2, while only eight holes are illustrated in FIG. 1.
A light emission element 3 and a light reception element 4 are disposed to oppose to each other in the same axis at one and the other side of the code disk 2 respectively. Upon rotation of the code disk 2, the light emitted from the light emission element 3 impinges upon the disk 2 and detected by the light reception element 4 when any one of the holes 2a comes to the detection position, that is on the above-mentioned axis of disposition of the light emission and light reception elements 3 and 4. Thus, pulses are produced from the light reception element 4 at a frequency corresponding to the revolution of the code disk 2.
In such a rotary encoder as described above, in order to detect the direction of rotation of the code disk 2, or to multiply the output signal, it is necessary to take out two kinds of output signals different in phase from each other. In order to obtain such two kinds of output signals, conventionally, two sets of light emission and light reception elements are used to perform detection at the two points A and B in FIG. 1. Further, conventionally, a phase shift plate 5 is used in order to obtain two outputs different in phase from each other at the points A and B respectively. The phase shift plate 5 is constituted by a fan-like fixed plate provided with holes 5a bored at a distance different from the array pitch of the detection holes 2a. Thus, the respective time points where any two of the detection holes 2a pass the detection points A and B directly above the holes 5a offset with each other. Thus, a phase difference as seen in FIG. 3 occurs between the detection outputs of the respective light reception elements 4 at the points A and B. In order to process the two outputs as shown in FIG. 3 as a trigonometric function and to perform the signal multilying processing by using the two outputs obtained at the points A and B, it is required that the phase shift between the outputs is (90.degree..times.n) (n being an integer). In the example of FIG. 3, the output obtained at the point B is behind by 90.degree., that is 1/4 of the period T, than the output obtained at the point A.
In the conventional rotary encoder, however, two outputs different in phase are taken out by setting the relative position between the holes 5a bored in the phase shift plate 5 and the detection holes 2a in the code disk 2 and therefore it is required to work the phase shift plate 5 with accuracy. Further, in fixing the phase shift plate 5, it is necessary to position the phase shift plate 5 relative to the code disk 2 with high accuracy, resulting in troublesome adjusting working. In addition to these disadvantages as described above, the conventional rotary encoder has a further disadvantage that the cost thereof is high and in the case where the disposing pitch of the detection holes 2a in the code disk 2 is changed, it is uneconomically required to form a new phase shift plate 5.